html,
body {
  width: 100%;
  min-height: 100%;
  background: linear-gradient(45deg, blue 8%, red);
}

form {
  width: 450px;
  position: absolute;
  left: 50%;
  top: 45%;
  transform: translate(-50%, -50%);
  padding: 30px;
  background: white;
}
